Tank urret rotation - Need help.

Hi guys,

for 3 three hours I try to rotate a tank turret now... And I only get it to smoothly look to the target. But the problem is how can I fix the Z axis of the turret after looking/slerping to the target?

Here is an example:

After this phase of rotation I want that the local rotation of the turret gets to the same level as the tank hull again. Can anyone help me please? I tried so many experiments on my own with this stupid tank :D

Here is my latest code experiment:

 // The Important snippet from the Weapon.cs Script
 //
 // Transform weaponHolder = <turret set via inspector>
 // The parent of the turret is the hull.
 
 Vector3 direction = target.transform.position - transform.position;
 Quaternion toRotation = Quaternion.LookRotation(direction);
 Quaternion newRotation = Quaternion.Slerp(weaponHolder.transform.rotation, toRotation, Time.deltaTime * 1.5);
 weaponHolder.rotation = newRotation;
 weaponHolder.localEulerAngles = new Vector3(weaponHolder.localEulerAngles.x,
                                     weaponHolder.localEulerAngles.y,
                                     weaponHolder.parent.localEulerAngles.z);